This invention relates to a process for producing an electrode of an alkaline battery and is specifically directed to a process for producing an electrode having high energy density, good mechanical strength and excellent reactivity of the active material by the use of an unsintered fluorocarbon resin as a binder.
In an enclosed type Ni-Cd battery, for example, it is an essential commercial requirement that the cadmium electrode has a charge-discharge life of at least 1,000 cycles, an energy density of at least about 500 mAH/cc and extremely good reactivity with an oxygen gas for the purpose of gas absorption inside the battery.
As an electrode satisfying these requirements, a sintered type electrode is generally known. The electrode consists of a porous substrate obtained by sintering a nickel powder and then impregnating the substrate with an active material.
In the sintered type electrode of the above-described type, however, the nickel powder forming the porous substrate is expensive. In addition, the porous substrate does not with certainty participate in the reaction from the standpoint of battery capacity so that the energy density obtainable is at the maximum about 600 mAH/cc. Moreover, it is a time-consuming and troublesome operation to form the porous substrate and to have the active material impregnated and supported inside pores of the substrate. For these reasons, the resulting electrode becomes necessarily expensive.
Apart from the abovementioned sintered type electrode, a paste type electrode also is known in the art.
The production process of this paste type electrode generally comprises kneading an active material powder into a paste form together with an aqueous solution of a paste, dissolving therein a natural or synthetic polymer paste material such as polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, polyethylene oxide and the like, coating the resulting paste onto an electrically conductive core body and drying the core to form the electrode.
In comparison with the sintered type electrode, the paste type electrode is more advantageous in that the number of production steps is less and since the major part of the electrode is formed by the active material which participates in the battery reaction, the obtainable energy density is by far higher, that is to say, up to about 700 mAH/cc. However, the paste type electrode is not free from drawbacks. Since the polymer paste used as the binder is swellable in water which is one of principal components of an electrolyte, the electrode does not have sufficient mechanical strength. Also, the polymer paste is oxidized inside the battery to form a carbonic acid ion in such an amount as to exert adverse influence on the battery reaction. Furthermore, since the binding mechanism of the polymer paste relies on a film-forming action, the active material is partially covered by the film, thereby lowering its reactivity.
As a method of improving these drawbacks, U.S. Pat. No. 3,630,781 discloses a novel production method of an electrode similar to the paste type electrode.
This method is characterized by the use of an unsintered fluorocarbon resin, e.g., tetrafluoroethylene resin (hereinafter referred to as PTFE), hexafluoropropylene resin, chlorotrifluorroethylen resin, vinylidene fluoride resin, and copolymer variations thereof, as a binder. More specifically, the method comprises dispersing and homogeneously mixing an active material powder in a water-soluble dispersion of the PTFE resin obtained by emulsion polymerization, irreversibly breaking said dispersion under conditions such that the PTFE resin is not sintered, applying a shear force so as to cause fiber-formation in the PTFE resin and thus form a rubbery mass having malleability in such a state where the active material and the residual water are retained inside the resulting network structure of the PTFE resin, forming a sheet from the rubbery mass by an ordinary calender method and attaching the sheet to an electrically conductive core to thereby obtain the contemplated electrode.
It is known that when a shear force is applied, the chain-like molecules of the abovementioned unsintered PTFE resin obtained by emulsion polymerization cause fiber-formation and entangle with one another to form a network structure and thus exhibit a certain kind of binding effect.
The PTFE resin has extremely good oxidation resistance and chemical resistance and does not form a carbonic acid ion which occurs when the aforementioned polymer paste is used. Since the binding action of the PTFE resin does not rely on the film-forming action as in the case of the polymer paste, the active material is maintained in good contact with the electrolyte as well as with the gas. In order to obtain the same mechanical strength for a given electrode, the weight required for the resin is about 1/2 and the volume is about 1/4 in comparison with the polymer paste (the density of the PTFE resin being about two times the polymer paste).
As mentioned above, the method of U.S. Pat. No. 3,630,781 is definitely advantageous when compared with the conventional paste type electrode.
In the method of U.S. Pat. No. 3,630,781, however, the active material powder is mixed with the aqueous dispersion of the PTFE resin and the dispersion is then irreversibly broken. Accordingly, in the sense of "mixing", the homogeneous state is established between the active material and the fluorocarbon resin, but from the viewpoint of the battery performance and mechanical strength, the excessive homogenity is not only meaningless but also involves a possible problem that a phenomenon similar to the film-forming action of the polymer paste can occur due to the water-repellency of the fluorocarbon resin.
In addition, the dispersion contains residual surfactant used for the purpose of keeping the water-repellent PTFE resin in the emulsified and dispersed state in water and residual catalyst used for the emulsion polymerization. In order to eliminate the adverse influence of these agents on battery performance, an additional step of removing these additives is necessary after formation of the electrode.
During the abovementioned production steps, it is necessary at the step of forming the rubbery mass to partially reduce the water content of the dispersion and set the residual water content to a predetermined level in order to apply the shear force. Since the removal of the water is carried out by such means as evaporation by heating, control is not easy. During the step of applying the shear force after removal of water, the energy loss of the shear force is great due to lubricative effect of the water, and it is difficult to form the network structure of the PTFE resin having a predetermined strength with a high level of accuracy.
Because of the viscosity of the fibers of the PTFE resin, the surface of the electrode thus finished is adherent. Hence, the active material is sometimes peeled off upon contact with other components, such as the separator during the assembly of the battery. For this reason, the adherent electrode is extremely difficult to handle from the aspect of assembly. It is of further significance that when an active material having high reactivity with water such as cadmium oxide is used as the starting material, cadmium oxide readily reacts with the water during the mixing step with the aqueous dispersion and such reaction affects adversely the battery performance.
In other words, as illustrated by the following reaction scheme; EQU CdO+H.sub.2 O.fwdarw.Cd(OH).sub.2
cadmium oxide readily reacts with the water whereby cadmium oxide having a large density (8.15 g/cm.sup.3) is converted to cadmium hydroxide (density=4.79 g/cm.sup.3). This increase in the volume takes place during the production of the electrode and consequently, the packed density per unit volume of the active material is reduce, thereby resulting in a decrease in the energy density. For this reason, the upper limit of the energy density is at the maximum about 750 mAH/cc in accordance with this method.
On the other hand, in addition to the aforementioned aqueous dispersion of the PTFE resin, an unsintered PTFE resin of such a type is also commercially available. In such a resin, detrimental components adversely affecting the battery performance such as the surfactant, the catalyst and the like are perfectly removed by aggregating the resin after the emulsion polymerization.
U.S. Pat. No. 3,898,099 proposes a production method of an electrode using an unsintered PTFE resin powder of the abovedescribed type.
The method of U.S. Pat. No. 3,898,099 is different from that of the abovementioned U.S. Pat. No. 3,630,781 in that it uses a PTFE resin powder and non-aqueous lubricant in place of the aqueous dispersion of the PTFE resin.
Since this method uses non-aqueous lubricant, it is possible to prevent cadmium oxide from being converted to cadmium hydroxide. Another advantage of this method is that the PTFE powder does not contain the additives affecting adversely the battery performance such as the surfactant, the catalyst, etc., thus the method does not require the additional step of removing these additives.
According to the method of U.S. Pat. No. 3,898,099, however, the non-aqueous lubricant is added in an excessive amount in order to obtain a homogeneous mixture of the active material and the PTFE resin powder. In removing the lubricant, therefore, it is difficult to control the residual liquid amount to a predetermined level. It is further necessary in this method to perfectly remove the lubricant after the formation of the electrode and for that purpose, drying is carried out. However, this drying treatment must be carried out at a temperature below the sintering temperature of the PTFE resin. Hence, the lubricant to be used is necessarily restricted to volatile lubricants such as mineral spirits, for example. This inevitably results in harm of organic solvents and inevitably requires use of a large-sized production apparatus.